The development of the petroleum industry in China has been oriented to seas and oceans gradually. Especially, the efforts have been strengthened continuously in exploration and development of deep-water oil and gas fields in the South China Sea region. Offshore well drilling, especially deep-water well drilling, is confronted with a much harsher operating environment than the onshore well drilling. A prominent problem is the narrow “safe density window” incurred by the existence of seawater. To realize safe deep-water well drilling, the pressure in the wellbore must be controlled within the safe density window. Therefore, to make the pressure distribution in the wellbore closer to the two environmental pressure distributions (seawater pressure distribution above the mud line, and formation pressure distribution below the mud line) confronted in deep-water well drilling, new techniques such as dual-gradient offshore well drilling have been developed gradually, and have exhibited their obvious advantages in applications.
The key point in the wellbore pressure control in deep-water dual-gradient drilling is to keep the pressure distribution in the marine riser above the subsea wellhead to be equal to seawater hydrostatic pressure distribution and keep the pressure distribution in the wellbore below the subsea wellhead to be equal to formation pressure distribution, especially and, in particular, to ensure that the subsea wellhead pressure is equal to the hydrostatic pressure of seawater at the seabed. To implement deep-water dual-gradient drilling, special devices, such as rotary flow divider, etc., are installed at the subsea wellhead, so as to divide the wellbore into two parts: the upper part above the subsea wellhead is the marine riser part, and the lower part below the subsea wellhead is the wellbore part. As a result, spatial division is realized. However, the hydraulic system of the wellbore is also isolated, i.e., the marine riser part above the subsea wellhead is filled with seawater, while the wellbore part below the subsea wellhead is filled with drilling fluid. In this case, the pressure in the upper part and the pressure in the lower part of the cyclone separator are not equal to each other, which means the subsea wellhead pressure below the cyclone separator is not equal to the hydrostatic pressure of seawater at the seabed. Consequently, the detection and control of the internal pressure in the wellbore part are more difficult, and the accuracy of pressure control in deep-water dual-gradient drilling is decreased.
In addition, owing to the fact the seawater depth varies with time during drilling, the hydrostatic pressure of seawater at the seabed also varies with time. The monitoring of the pressure at the seabed is difficult owing to the existence of seawater, further affecting the accuracy of wellbore pressure control in dual-gradient drilling. Hence, it is necessary to design a device that can monitor the pressure at the seabed, keep the subsea wellhead pressure equal to the hydrostatic pressure of seawater at the seabed in real time and provide parameters for smooth operation of the subsea mud pump, with reference to the environmental characteristics of the deep-water well drilling.